New research

Insects could restrict how well trees absorb and
store carbon in the future, according to a new study. In
experiments simulating carbon dioxide levels in 2050, insects
munched their way through almost double the number of leaves than
under current conditions.

With fewer leaves, the trees are likely to
become less effective at absorbing carbon dioxide from the air, the
researchers say.

Carbon dioxide fertilisation

Trees are the biggestcarbon sinkon land. Through a
process known as photosynthesis, trees convert carbon dioxide,
water and sunlight into the fuel they need to grow, locking up
carbon in their trunks and branches as living biomass.

Researchsuggests that when there's more carbon dioxide in the air,
trees grow more quickly because the rate of photosynthesis speeds
up. This is called 'carbon dioxide fertilisation'.

Scientistsexpectthat as human-caused carbon dioxide
emissions increase, forests will absorb and store more carbon,
assuming they have enough water and nutrients to grow.

But the results of a three-year experiment, just
published inNature
Plants, suggests that insects, such as
caterpillars, may make it harder for trees to absorb this extra
carbon by munching their way through the trees' leaves. With fewer
leaves to absorb sunlight, the trees can't photosynthesise as much,
and they absorb less carbon dioxide from the air.

'Double whammy'

The researchers set up their experiment at
theAspen FACE
facility(Free-Air Carbon Dioxide Enrichment) in
Wisconsin in the northern US. At the facility, birch and aspen
trees are surrounded by vertical vents that release different gases
to simulate future conditions.

This month will see the civil war in Syria reach
an inauspicious fourth birthday. Since the uprising in 2011,
over220,000 people have been killedand almost
half the Syrian population have fled their homes.

Evidence suggests the conflict was triggered by
a complex mix ofsocial, political, economicandenvironmentalfactors. But new research
finds that human-caused climate change could also have had an
influence.

The study, published in theProceedings
of the National Academy of Sciences, suggests a
severe drought that began in 2006 was a catalyst for the conflict,
and that climate change has made such droughts in the region more
than twice as likely.

Syrian conflict

On 15th March 2011, Syrian security
servicesopened
fireon pro-democracy protesters in the southern
city of Dara'a, killing several people. The unrest that followed
spread throughout the country over the ensuing months, and
byFebruary 2012, Syria had descended into
civil war.

Astudypublished last year found that a
multi-year drought contributed to food shortages, urban migration,
and unemployment in the run up to the conflict.

Now the new study says the drought had a
catalytic effect on the unrest in Syria, and human-caused climate
change has made the chances of such a severe drought between two
and three times more likely.

Prof Richard
Seager, a climate scientist at Columbia University
and co-author on the new study, explains:

"We're not saying
drought caused the war. We're saying that added to all the other
stressors, it helped kick things over the threshold into open
conflict. And a drought of that severity was made much more likely
by the ongoing human-driven drying of that region."

Multi-year drought

Syria sits in a band of relatively moist and productive land in
the Middle East, known as the Fertile Crescent. But between 2006
and 2010, the region was hit by the worst multiyear drought
since 1940.

Syria gets almost all of its rain during its
six-month winter, from November to April. In 2007-08, winter
rainfall across Syriafell by a third, with some areas receiving
no rain at all. The winter was the driest in the observed record,
the researchers say.

The decreasing rainfall (shown in the top graph
below) combined with rising temperatures (second graph) resulted in
a decline in soil moisture (third graph), the researchers say. This
had dramatic consequences for Syrian agriculture.

Scientists know that greenhouse gases in the
atmosphere cause the Earth to warm. But measuring exactly how much
heat they trap is harder than you might think.

Previous
studies using satellites have established that more heat is
entering the atmosphere than leaving it. But a new study goes a
step further and directly measures the amount of warming greenhouse
gases are producing at Earth's surface.

The paper provides the critical link between
rising carbon dioxide concentrations and the extra energy trapped
in the climate system, the researchers say.

Greenhouse effect

Joseph Fourier first suggested in the 1820s that
gases in the Earth's atmosphere trap heat and help keep the planet
warm, coining the termgreenhouse effect. Physicist John Tyndall
later extended the theory by identifying the gases, such as carbon
dioxide and methane, that were responsible for the
warming.

But while those studies show a widening gap
between the energy reaching and leaving Earth, they are unable to
directly measure how much warming greenhouse gases are causing at a
particular point in time. New research, published today inNature, shows how scientists have directly
been able to measure the warming effect of greenhouse gases at
Earth's surface.

Measuring energy

The researchers used a set of instruments to
take thousands of measurements at the Earth's surface. The
instruments record the longwave energy that is re-emitted by
greenhouse gases back towards the Earth's surface, which causes the
warming.

Making these sorts of measurements on the ground
is difficult, says lead authorDr
Daniel Feldman, a geological scientist at the
Lawrence Berkeley National Laboratory in the US. With weather
systems passing overhead, and temperatures and humidity changing
frequently, it's tricky to take energy measurements without other
factors getting in the way.

To overcome this problem, the researchers
measured temperature and water vapour at the same locations so that
their influence on warming could be eliminated from the
calculations, leaving just the impact of greenhouse
gases.

The scientists used data from 2000 to 2010,
collected from two sites in the US: the southern Great Plains and
northern Alaska. They chose these sites because of their very
different climates, says Feldman. This meant the researchers could
investigate both a mid-latitude and a high-latitude
location.